831
FXUS62 KCHS 261254
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
854 AM EDT Thu Sep 26 2024

...TORNADO WATCH IN EFFECT FOR ALL LAND AREAS AND THE ADJACENT
COASTAL WATERS THROUGH 9 PM...

.SYNOPSIS...
Hurricane Helene is forecast to move inland across portions of
Florida and Georgia later today into Friday. Multiple impacts
are expected across our area today and tonight. Drier weather
then arrives later Friday and persists into next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today and Tonight: Aloft, a ridge will be centered far off the
Southeast Coast while a large cutoff low remains over the middle
Mississippi River Valley. At the sfc, high pressure extending south
along the East Coast and into the Southeast will retreat offshore in
advance of Hurricane Helene, which is forecast to quickly track
north across the eastern Gulf of Mexico during the day, then make
landfall as a Major Hurricane across the Florida Panhandle around
nightfall. Given the speed of Helene, strong winds are expected
further inland than typical. Additionally, wind fields will expand
as Helene gains latitude, likely producing impacts well east of its
center, which is forecast to track north across Central Georgia. For
this reason, a Tropical Storm Warning remains in effect for the
entire forecast area today and through the night, including across
nearby coastal waters. The latest forecast will keep Southeast
Georgia and Southeast South Carolina along Helene`s eastern side
throughout the day and night, bringing multiple hazards to the
immediate area. For full details, please refer to weather.gov/chs
and the Hurricane Local Statement (HLS).

Tornadoes: Perhaps the greatest concern in regards to impacts
associated with Helene will be the risk for tropical tornadoes
across the entire area, especially given the local area remains
situated in the favorable northeast quadrant of Helene for quite
some time today and tonight. Guidance continues to support
conditions favorable for tornadoes, with long and looping hodographs,
ample buoyancy along a northward shifting CAPE axis (SBCAPE
1500-2000 J/kg) extending from the nearby Atlantic and onshore,
sfc temps in the upper 70s to lower 80s, and 0-3 km SRH values
near 300-400 m2/s2. Additionally, the latest HREF run gives a
fairly strong indication of updraft helicity tracks supportive
of tornadic waterspouts that shift onshore, coinciding with
rainbands associated with Helene this afternoon and the night.
The latest SPC Day 1 Convective Outlook does include an Enhanced
Risk for severe weather across the majority of the local
forecast area, with 10% tornado probabilities driving the bulk
of the risk. Have a tornado plan in place in case a warning is
issued, but keep in mind that tornadoes of this type can form
with little advance warning and move very quickly (potentially
in excess of 45 to 55 mph).

Winds: A Tropical Storm Warning is in effect for the entire forecast
area. Based on the 5 AM NHC advisory, the Earliest Reasonable
Arrival Time of Tropical Storm Forecast Winds (39+ mph) in our area
is mid to late afternoon across Georgia, and early evening across
South Carolina. The Most Likely Arrival of these winds is early
evening across Georgia and during Thursday night across South
Carolina. Gusty winds as high as 40 to 60 mph could produce downed
and/or uprooted trees, isolated to scattered power outages locally,
with greatest impacts anticipated during the night.

Flooding Rainfall: Multiple rounds of showers/rainbands in
association with Helene will begin to impact the area this morning,
initially across Georgia counties, then spread north across South
Carolina counties during the afternoon. These showers along with a
few thunderstorms will then persist through the night, before
considerably drier air wraps around the southern edge of departing
Helene and overtakes the entire forecast area by around daybreak
Friday.

Rainfall totals are mainly in the 1.5 to 3 inch range, but with
locally higher amounts possible in heavy rainbands. The risk
currently does not support the issuance of a Flood Watch, especially
with dry soils in many places per NASA Sport guidance. That said,
area rivers and creeks will rise in response to the rainfall, with
some flooding possible, even after the rains come to an end.

Storm Surge: The latest forecast has storm surge values in the 1 to
3 foot range. As such, coastal flood products might be required at a
later time. Additional information can be found in the Tides/Coastal
Flooding section below.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY/...
Strong upper low lingers near the Tennessee Valley through the end
of the week. Attendant surface low/remnants of Helene remain quasi-
stacked despite trending more baroclinic, but steadily fills. A few
lingering showers Friday morning will quickly diminish by midday
Friday, with strong subsidence and mainly dry conditions persisting
across SE SC/GA thereafter. Breezy S to SW winds Friday morning
trend moderate by Friday afternoon, with humid conditions persisting
into the weekend and temps remaining very close to normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
A mid and upper low initially over the Ohio Valley will eventually
open into a wave as it reaches the Mid-Atlantic coast Tuesday.
Forcing for ascent remains north of the area, with subsidence aloft
persisting locally, keeping a mainly dry forecast in place. At the
surface, weak ridging persists early in the week, with a cold front
(which appears mainly dry) crossing mid to late week. Temps
generally near to a few degrees below normal through the period.

&&

.AVIATION /13Z THURSDAY THROUGH MONDAY/...
VFR conditions will persist through about 14-15Z at CHS and JZI
terminals. Otherwise, showers and potentially a few thunderstorms
will continue to spread north across the region today, resulting in
at least prevailing MVFR conditions at CHS/JZI/SAV terminals through
12Z Friday. There is some potential for IFR conditions to develop at
all terminals after 00Z Friday, but currently low-end MVFR
conditions are forecast. In addition to moderate and possibly heavy
showers, wind speeds will noticeably increase this afternoon and
especially late afternoon into the evening as Helene gains latitude
and passes inland. By 02-04Z Friday, southeast winds gusting up to
35-40 kt should prevail at all terminals, beginning at the SAV
terminal first. Gusts could top out near 45-50 kt at SAV tonight by
the end of the TAF period. LLWS will likely be needed at all
terminals at some point starting around 00Z Friday. Future TAF
issuances will likely address this issue.

Extended Aviation Outlook:
Extended Aviation Outlook: Prevailing VFR, though with
persistent elevated low level moisture and a strong subsidence
inversion, will need to monitor for morning radiation fog and
afternoon MVFR strato- cu potential through the long term.

&&

.MARINE...
Today and Tonight: Dangerous marine conditions will occur as Helene
strengthens across the eastern Gulf of Mexico during the day, then
moves onshore across the Florida Panhandle around nightfall and
continues north across Central Georgia during the night. Helene is a
large tropical cyclone with wind fields that are expected to expand
wide as the cyclone gains latitude, favoring impacts well to the
east. Tropical Storm Warnings will remain in effect for all local
waters as a result. Winds as high as 45-50 kt will start impacting
the Georgia waters this evening, then spread into the South Carolina
waters overnight. Seas will build as high as 10-15 feet within 20 nm
from shore, and up to 17 feet across the outer Georgia waters
tonight.

Mariners are also alerted to the risk for waterspouts this afternoon
and through much of the night. Some of these will be tornadic
waterspouts, moving much quicker and producing much stronger winds
than most waterspouts in our area.

Very large medium period swell associated with the large wind field
of Helene peaks Friday morning, gradually subsiding into the
weekend. Near Tropical Storm Force winds early Friday morning will
subside to more moderate winds by Friday afternoon as well. Light to
moderate winds persist Friday night into early next week as high
pressure moves overhead and more seasonable 2-3 ft seas prevail.

Rip Currents and High Surf: Conditions at the beach will be
dangerous today through Friday, as winds and seas build in
response to Hurricane Helene tracking across the northeast Gulf
of Mexico and into the interior of the Southeast. A High Risk
of Rip Currents will be in effect for all beaches both days, and
breakers will reach at least 5 feet in height starting this
afternoon and persisting through at least Friday afternoon or
evening. Areas of significant beach erosion are also likely during
this time.

&&

.TIDES/COASTAL FLOODING...
Strengthening onshore winds will lead to a continuation of the
elevated tides today, and minor coastal flooding is likely along the
Charleston and Colleton County coasts with the afternoon high tide.
The threat for coastal flooding is then expected for the entire
coast through tonight and early Friday due to impacts from Hurricane
Helene. Storm surge inundation of 1 to 3 ft is possible along the
coast of southeast Georgia and Southern South Carolina. Keep in mind
that if the higher surge and/or heavy rains occur near or during the
time of the high tides, the coastal flooding could be worse.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...Tropical Storm Warning for GAZ087-088-099>101-114>119-137>141.
     High Risk for Rip Currents until 8 PM EDT this evening for GAZ117-
     119-139-141.
     High Surf Advisory from 2 PM this afternoon to 5 PM EDT Friday
     for GAZ117-119-139-141.
SC...Tropical Storm Warning for SCZ040-042>045-047>052.
     High Risk for Rip Currents until 8 PM EDT this evening for
     SCZ048>051.
     High Surf Advisory from 5 PM this afternoon to 8 PM EDT Friday
     for SCZ048>051.
MARINE...Tropical Storm Warning for AMZ330-350-352-354-374.
&&

$$

NEAR TERM...
SHORT TERM...CEB
LONG TERM...CEB
AVIATION...CEB/DPB
MARINE...